The present invention generally relates to serial printers, and more particularly to a serial printer which prints an image on a recording sheet using the electrophotography technique by forming a toner image on the recording sheet by a recording drum.
Recently, in order to meet the demands to reduce both the cost and size of printers using the electrophotography technique, serial printers provided with a carriage that carries out the printing using the electrophotography technique have been developed. According to such a serial printer, the carriage is moved on a transfer unit in a direction perpendicular to a transport direction of the recording sheet so as to transfer an image on the recording sheet, and the transferred image on the recording sheet is fixed by a fixing unit which has the form of a roller arranged in the transport direction. There are now demands to improve the printing quality of such serial printers.
FIGS. 1A and 1B show the construction of a first conventional serial printer. FIG. 1A shows a plan view of a part of this first conventional serial printer, and FIG. 1B shows a cross section of a carriage of this first conventional serial printer.
A serial printer 11 shown in FIGS. 1A and 1B is proposed in a Japanese Laid-Open Patent Application No. 61-152463, for example. A shaft 14 is arranged parallel to transport rollers 13a and 13b which transport a recording sheet 12 in a transport direction shown by arrow T in FIG. 1A. A carriage 15 is movable in a direction perpendicular to the transport direction under guidance of the shaft 14, and this carriage 15 is driven by a driving motor (not shown). A fixing unit 16 having a width greater than the width of the recording sheet 12 is fixedly arranged on the downstream side of transport roller 13a in the transport direction. A transfer unit 17 is arranged under the recording sheet 12 along a moving direction of the carriage 15.
An image bearing member 21 is provided in the carriage 15, and this image bearing member 21 rotates at a peripheral speed in synchronism with the movement of the carriage 15. The surface of the image bearing member 21 is uniformly charged by a charger 22, and an electrostatic latent image is formed on the surface of the image bearing member 21 by an exposing unit 23. The electrostatic latent image on the surface of the image bearing member 21 is visualized into a toner image by a developing roller 26 using a toner 25 of a developing unit 24. The toner image on the image bearing member 21 is transferred onto the recording sheet 12 by the transfer unit 17 which confronts the image bearing member 21 via the recording sheet 12, and the transferred image is fixed when it is transported to the position of the fixing unit 16.
FIG. 2 shows the construction of a carriage provided with a fixing unit. This carriage is proposed in a Japanese Laid-Open Utility Model Application No. 61-145849, for example. A fixing unit 27 is provided inside a carriage 15. A cleaner 30 cleans the surface of the image bearing member 21 after the printing ends so as to remove the residual toner.
A fixing roller 28 which rotates in the same direction as the image bearing member 21 is provided in the fixing unit 27. A heat source 29 such as a halogen lamp is provided within the fixing roller 28 as a heating means. This fixing roller 28 is preheated to a predetermined temperature by the heat source 29 prior to the printing operation, and the temperature during the printing is controlled by detecting the temperature by a temperature detector (not shown) such as a thermistor. In other words, the fixing unit 27 is moved together with the image bearing member 21 and carries out the image fixing immediately after the image transfer.
The image transfer by the transfer unit 17 is carried out by applying a predetermined voltage across the transfer unit 17 and the image bearing member 21. Hence, a conductive member such as conductive rubber is formed on a substrate which forms the transfer unit 17.
FIGS. 3, 4A and 4B show a second conventional serial printer. FIG. 3 shows a plan view of this second conventional serial printer. FIG. 4A shows a cross section along a line 4A--4A in FIG. 3, and FIG. 4B shows a cross section along a line 4B--4B in FIG. 3. A carriage 34 of a serial printer 31 is provided with a processing part 32 and a fixing unit 33. This carriage 34 is guided by guide shafts 35a and 35b, and is moved in a direction shown by arrow P in FIG. 4A, perpendicular to the transport direction, shown by arrow T in FIG. 3, of a recording sheet by a carrier motor 36 via a belt 37.
A transfer unit (transfer platen) 39 is arranged under the carriage 34 along the moving direction of the carriage 34. A transport roller 40 is provided on the upstream side of the transfer unit 39 along the transport direction T, and transports the recording sheet 38 in the transport direction indicated by the arrow T in FIGS. 3 and 4B.
In FIG. 4A, the processing part 32 of the carriage 34 is provided with a recording drum 21, similarly to the case shown in FIG. 2. The recording drum 21 rotates on the recording sheet 38 which is on the transfer unit 39 in synchronism with the movement of the carriage 34 through a rack-and-pinion mechanism.
The surface of the recording drum 21 is uniformly charged by the charger 22, and an electrostatic latent image is formed on this surface by the exposing unit 23. The electrostatic latent image is visualized as a toner image by the developing roller 26 using the toner 25 within the developing unit 24. The toner image formed on the surface of the recording drum 21 is transferred onto the recording sheet 38 by applying a predetermined voltage across the recording drum 21 and the transfer unit 39 which confronts the recording drum 21 via the recording sheet 38. After the image transfer, the surface of the recording drum 21 is discharged, and the residual toner after the discharge is removed by the cleaner 30. The fixing unit 33 is provided with the fixing roller 28, and the temperature of the fixing roller 28 is controlled to a predetermined temperature by a thermistor 28a.
In FIG. 4A, the recording sheet 38 is transported between the recording drum 21 and the transfer unit 39 by the transport roller 40.
Although not shown, a fixing unit is provided within the carriage, and this fixing unit is provided with a non-contact type heat source that irradiates a heat ray on the recording sheet. The non-contact type heat source may be a halogen lamp using infrared ray or, a xenon lamp using flash fixing. According to the serial printer proposed in a Japanese Laid-Open Patent Application No. 56-77167, for example, the image bearing member (recording drum) recedes from the recording sheet about a guide shaft of the carriage when transporting the recording sheet.
Next, a description will be given of a third conventional serial printer, by referring to FIGS. 5A through 5C. FIG. 5A shows a plan view of the third conventional serial printer. FIG. 5B shows a cross section along a line 5B--5B in FIG. 5A, and FIG. 5C shows a cross section along a line 5C--5C in FIG. 5A.
In a serial printer 31.sub.A shown in FIGS. 5A through 5C, the recording drum 21 and a fixing roller 28a are respectively rotated by motors 41 and 42 via belts 41a and 42a at peripheral speeds synchronized to the movement of the carriage 34. Otherwise, the serial printer 31.sub.A is basically the same as that shown in FIGS. 3, 4A and 4B.
FIG. 6A and 6B are diagrams for explaining the printing operation of the serial printer 31.sub.A. As shown in FIGS. 6A and 6B, the recording sheet 38 is pinched between the transfer unit 39 and the recording drum 21 and the fixing roller 28a, and the recording drum 21 and the fixing roller 28a rotate so as to travel along sheet 38 in direction perpendicular to the transport direction T of the recording sheet 38, thereby to carry out the printing and fixing of a single line. The recording drum 21 and the fixing roller 28a return to an original position (home position) after the printing and fixing of the single line. Then, the recording sheet 38 is transported by a predetermined amount, and the printing and fixing are repeated thereafter in a similar manner for the subsequent lines.
In this case, as shown in FIG. 6B, a fixed area 38.sub.b1, indicated by vertical dashed lines, is larger than a first printed area 38.sub.a1, indicated by cross-hatching. When a second printed area 38.sub.a2 indicated by cross-hatching, is formed in continuous relationship with the first printed area 38.sub.a1, a fixed area 38.sub.b2 indicated by horizontal dashed lines is formed, and this fixed area 38.sub.b2 is larger than the second printed area 38.sub.a2. Such a process of forming the printed area and the fixed area is repeated in a similar manner.
However, according to the first conventional serial printer shown in FIGS. 1A and 1B, the fixing unit 16 is provided outside the carriage 15. For this reason, there are problems in that the size of the serial printer becomes large, and the serial printer becomes expensive because the fixing roller must have a length at least amounting to the width of the line to be printed on the recording sheet 12. In addition, because the length of the fixing roller amounts to the width of the recording sheet 12, even though the recording sheet 12 is transported intermittently, the contact time between the fixing roller and the recording sheet 12 having the toner image transferred thereon becomes greatly different between the part where the recording sheet 12 is stationary and the part where the recording sheet 12 is transported. This difference in the contact time introduces inconsistent fixing, presenting a problem in that the image quality of the printed image on the recording sheet 12 becomes poor.
On the other hand, according to the carriage shown in FIG. 2, the recording drum 21 and the fixing roller 28 constantly make contact with the recording sheet 12 or the transfer unit 17. For this reason, there is a problem in that the residual toner on the recording drum 21 may be transferred to parts other than the recording sheet 12, causing contamination. In addition, the toner 25 or the like on the transfer unit 17 may adhere on the fixing roller 28 and become fixed on the recording sheet 12, and in such a case, there is a problem in that the image quality of the image printed on the recording sheet 12 becomes poor.
Furthermore, according to the serial printer proposed in the Japanese Laid-Open Patent Application No. 56-77167, it is difficult to apply a sufficient amount of heat on the recording sheet when carrying out the fixing by the fixing unit using the non-contact type heat source. As a result, there is a problem in that a satisfactory fixing cannot be guaranteed. In this case, the halogen lamp, which heats the toner by infrared ray, requires an extremely long time for the fixing due to the low energy density, and in addition, there is a danger in that a fire may occur if a jam of the recording sheet is generated. On the other hand, according to the flash fixing using the xenon lamp, there are problems in that a large condenser becomes necessary, the toner is scattered, unpleasant noise is generated, and continuous irradiation is difficult in the case of the serial printer. Moreover, in either case where the halogen lamp or the xenon lamp is used, the lamp is be arranged close to the recording drum on the carriage, and there is a problem in that it is difficult to block the ray from the lamp with respect to the recording drum.
According to the second conventional serial printer shown in FIGS. 3, 4A and 4B, the carriage 34 provided with the process part 32 and the fixing unit 33 is guided by the guide shafts 35a and 35b. For this reason, there are problems in that it is difficult to independently attend to the maintenance of the processing part 32 and the fixing unit or to replace the consumption parts of the serial printer 31 such as the recording drum 21. In addition, since the fixing roller 28 is heated to a high temperature, there are problems in that the service life of the fixing roller 28 is short, and the high temperature gives undesirable effects on the processing part 32 causing deterioration of the image quality. Furthermore, the respective distances between the recording sheet 38 and the recording drum 21 and the fixing roller 28 do not become constant because of the unevenness of the guide shafts 35a and 35b, the warping of the fixing roller 28 due to the heating and pressure, and the warping of the transfer unit along the carriage moving direction, and there was a problem in that satisfactory fixing cannot be made due to poor transfer and insufficient pressure.
On the other hand, according to the serial printer 31.sub.A shown in FIGS. 5A through 5C, the fixing width is greater than the printing width, and a non-printed region on the recording sheet 38 is also heated by the fixing unit 33. As a result, both heated parts and non-heated parts are generated within the non-printing region on the recording sheet 38, and the moisture absorbency of the recording sheet 38 changes due to the heating. The change in the moisture absorbency of the recording sheet 38 causes the resistance of the recording sheet 38 to become non-uniform, and there was a problem in that the image quality becomes poor due to the non-uniform resistance of the recording sheet 38. In addition, since the fixing roller 28a is rotated by the motor 42, there were problems in that the carriage 34 becomes large and heavy, thereby increasing both the size and cost of the serial printer 31.sub.A.